Magnetic recording discs are composed of a lined disc container, called a cartridge case (i.e., a cartridge or a jacket), having windows, e.g., a magnetic head (i.e., a magnetic head-inserting gate) window and a hole for disc rotation-driving, and a flexible magnetic disc rotably set in the lined container.
The liner of the disc container serves for smooth rotation of the flexible magnetic disc and for cleaning the surface of magnetic layer. The liner, single-layered or multi-layered, is made of nonwoven fabric mainly comprising, for example, rayon fiber, rayon/polypropylene mixed fiber, polyethylene terephthalate fiber, acrylic/polyester mixed fiber, acrylic/rayon mixed fiber, etc. bound by heat-fusing or with an adhesive and is fixed to the inner wall of a disc container. The nonwoven fabric usually contains therein a lubricant, a dispersant, and an antistatic agents e.g., polyoxyethylene, a glycol fatty acid ester or a sodium alkylsulfate, each in an amount of from 0.1 to 0.5% by weight by impregnation.
From the viewpoint of magnetic layer surface cleaning properties, nonwoven fabric mainly comprising rayon fiber is excellent as disclosed in JP-A-U-62-15172 (the term "JP-A-U" as used herein means an "unexamined published Japanese utility model application"). Besides, nonwoven fabric mainly comprising rayon fiber is less causative of damage to the surface of a magnetic layer than other fibers.
However, rayon fiber has a high water (or moisture) absorption which reaches about 27 to 28% in a high humidity environment of 95% RH (relative humidity). Therefore, a liner comprising rayon fiber expands in thickness on moisture absorption and increases its contact pressure against a magnetic disc, resulting in a rise of the running torque.
With respect to a method for obtaining a liner having nonwoven fabric mainly comprising rayon fiber on the side in contact with a magnetic layer, a liner comprising nonwoven fabric of 100% by weight rayon fiber has been proposed. In formation of nonwoven fabric, because a conventional heat-fusing process is not applicable to 100% rayon nonwoven fabric which is not thermoplastic, 100% rayon nonwoven fabric has been prepared by a latex process using an adhesive, a water stream interlocking process using a water jet stream, and the like.
However, the latex process is disadvantageous in that the adhesive, such as SBR adhesives or NBR adhesives. tends to fall off (or peel off) and adhere to a magnetic layer to cause dropout or damage to the surface of a magnetic layer. Such tendency is conspicuous particularly under a high temperature environment.
On the other hand, the water stream interlocking process gives only weak interlocking to fibers. The nonwoven fabric prepared by this process, when applied as a liner, suffers from an increased frequency of fall-off of fibers during running. As a result, cases are met in which temporary dropout increases or fallen fibers squeeze themselves between a head and a magnetic layer to cause scratches on the magnetic layer.
U.S. Pat. No. 4,610,352 (corresponding to JP-A-61- 258057) discloses nonwoven fabric prepared by heat-fusing of rayon fiber as an outer layer and a nylon fiber as an inner layer. However, nylon fiber has a water (or moisture) absorption of about 8 to 9% in a 95% RH environment, and the rayon/nylon double layered nonwoven fabric causes an increase of torque similarly to 100% rayon nonwoven fabric.
In fabrication (i.e., working) of nonwoven fabric into a liner for a disc container, the fibers are generally bonded to each other with an adhesive or by heat-fusing so that the fibers may not fall off to cause dropout, and then punched out into a specific shape. Since an adhesive used for fiber bonding frequently comes off nonwoven fabric to cause dropout, a liner is usually formed by heat-fusing of nonwoven fabric. Further, it is important for nonwoven fabric as a liner to hardly produce fiber waste causing dropout, on punching.
Of nonwoven fibers, polypropylene fiber is excellent in heat-fusibility and fabricability (i.e., workability), has satisfactory dimensional stability owing to small hygroscopicity, and is relatively cheap so that it has been widely employed as described, e.g., in JP-A-U-60-23969, JP-A-60-179944, and JP-A-62-257625 (the term "JP-A" as used herein means an "unexamined published Japanese patent application").
Hence, nonwoven fabric comprising mixed fiber of rayon and polypropylene has been proposed as a liner manifestating the above-mentioned advantages of polypropylene fiber and the excellent cleaning properties of rayon fiber, as disclosed in JP-A-62-257625. However, conventionally proposed nonwoven fabric of rayon/polypropylene mixed fiber, such as "149-303" produced by Veratec Co., has a disadvantage that when the nonwoven fabric is heat-fused under pressure and heat through calendar rolls, a part of polypropylene fiber forms small particles, which afterward not only fall off the nonwoven fabric to cause dropout but also scratch the surface of a magnetic layer.
Other known nonwoven fabric, such as rayon/polyester mixed nonwoven fabric "149-188" produced by Veratec Co., Ltd., is less fabricable than polypropylene nonwoven fabric, causing a considerable (i.e., workable) reduction in working life of a cutter used for fabrication or leaving cutting dust (fiber waste) in a liner sheet to cause dropout.
Three-layered nonwoven fabric using nylon fiber, such as "149-246" produced by Veratec Co., Ltd. (composed of inner and outer layers of rayon fiber and an intermediate layer of nylon fiber) undergoes large dimensional changes with changes in humidity because rayon fiber and nylon fiber both have a high moisture (or water) absorption as described above, causing a great increase in torque. When applied to a floppy disc drive designed to have a low motor torque, a magnetic recording disc sometimes fails to rotate.
On the other hand, with the recent increase of demand for improved recording density of magnetic recording media, an increase in recording capacity from 1 MB to 4 MB and further to 10 MB or even higher has been demanded. To meet the demand, study has been given to a ferromagnetic metal powder mainly comprising Fe, Ni or Co or a tabular hexagonal ferrite powder such as barium ferrite for use as a ferromagnetic powder in magnetic recording discs.
That is, an increased recording density can be obtained by using extremely small particles of these ferromagnetic powders, i.e., a ferromagnetic metal powder having a specific surface area (S.sub.BET) Of at least 30 m.sup.2 /g and a crystallite size of not more than 300 .ANG. as measured by X-ray diffractometry, or a tabular hexagonal ferrite powder having a specific surface area of at least 25 m.sup.2 /g, an tabular ratio of from 2 to 6, and a length in the long axis of from about 0.02 to 1.0 .mu.m. According as a recording density is thus increased, the above-mentioned problem of dropout has become more acute.